This paper reports an investigation of the influence of carbonitriding conditions for 20MnCr5 low alloy steel. Three gaseous carbonitriding conditions were investigated based on different carbon and nitrogen potentials to attain varying levels of carbon between 0.62 and 0.93% mass, whereas for nitrogen between 0.19 and 0.26% mass at the surface. Analysis of retained austenite and residual stress distributions was conducted using X-ray diffraction technique. The effective case depth varied between 900 and 1200 µm. The case microstructures were characterized by varying proportions of retained austenite and martensite, while the core contained essentially bainitic microstructures. The maximum amount of retained austenite which occurred at a depth of 50 µm from the subsurface ranged between 30 and 70% mass and significantly influenced the level of surface micro-hardness whereas the core hardness remaining relatively constant at 450 HV1. High values of residual stresses in martensite phase were observed. The signs, magnitudes, distributions and location of maximum compressive residual stresses were highly influenced by the maximum fraction of retained austenite. Retained austenite of 30%, 50% and 70% mass at the surface lead to peak compressive residue stresses of -280, -227, and -202 MPa at depths of 555, 704, and 890 μm, respectively.
Keywords: Carbonitriding, retained austenite, martensite, residual stress, XRD.
Simulation of transient heat transfer and phase transformation in Laser Beam Welding for low alloy steel and studying its influences on the welding residual stresses
